The present invention relates to an exhaust purifier for a diesel engine.
The exhaust gas (hereinafter referred to as “exhaust”) emitted from a diesel engine contains particulate matter (hereinafter referred to as PM). The use of a particulate filter (hereinafter referred to as filter) in an exhaust system for elimination of the PM is well known in the prior art. However, deposition of the PM clogs the filter and lowers the output of the diesel engine. In order to resolve the problem of PM deposition, the filter is heated to a predetermined temperature (approximately 650° C. (hereinafter referred to as regeneration temperature)) to oxidize (burn) the PM deposited in the filter and regenerate the filter.
The amount of air actually drawn into the engine decreases at high altitudes due to the low air density. In such a case, the amount of fuel injected into the engine is controlled so as to be reduced. This lowers the temperature of the exhaust. Thus, the exhaust purifier may not be sufficiently heated. Japanese Laid-Open Patent Publication No. 2005-016396 describes a technique for solving such a problem in which the intake air amount is increased when driving a vehicle from a low altitude to a high altitude.
Generally, the idle speed of an eight cylinder engine is lower than that of a four cylinder engine to improve fuel efficiency. However, when driving the vehicle while regenerating the exhaust purifier, if the engine starts to idle, the intake air decreases. As a result, for example, the balance between the heat generated by PM combustion and the heat absorbed by air cannot be maintained thereby causing overshoot (hereinafter referred to as deceleration OT). Thus, at least a predetermined amount of intake air must be ensured when controlling the temperature increase of the filter.
In a gasoline engine, a predetermined amount of intake air is ensured by widely opening the throttle valve. In a diesel engine, the necessary quantity of intake air is ensured even when the idle speed is lowered as long as the engine is running under a normal pressure environment. However, under a low pressure environment, the intake air amount may not be ensured even by correcting the opening of the throttle valve.